1. Field of the Invention
The present invention relates to an acceleration sensor to detect an acceleration, and more particularly to an acceleration sensor used, for example, for an airbag in an automobile.
2. Description of the Related Art
An acceleration sensor using a piezoelectric body has conventionally been used for detecting the acceleration in an airbag in an automobile and the like. An example of this type of acceleration sensor is disclosed in Japanese Laid-Open Patent Publication No. 5-322915. FIGS. 14 and 15 are each a perspective view illustrating the conventional acceleration sensor.
An acceleration sensor 51 has a structure in which an acceleration detection element 53 is mounted on a hybrid IC substrate 52. The hybrid IC substrate 52 is fixed to an insulating base substrate 54. A metal case 55 in a shape which opens downward is mounted from above onto a multilayered body formed of the hybrid IC substrate 52 and the base substrate 54, and the acceleration detection element 53 is enclosed in the internal space.
Signal pickup electrodes 56 and 57, which are electrically connected to the acceleration detection element 53, are formed on the hybrid IC substrate 52. The acceleration detection element 53 includes first and second piezoelectric bodies 58 and 59. The first and second piezoelectric bodies 58 and 59 are both formed of a lead zirconate titanate type piezoelectric ceramic, and are polarized in the direction of the arrows A1 and A2. That is, both the first and second piezoelectric bodies 58 and 59 are polarized so that the polarization directions of the first and second piezoelectric bodies 58 and 59 become parallel to the main surface of the hybrid IC substrate 52, but oppose each other.
Electrodes (not shown) are formed on the top surface and the bottom surface of the piezoelectric bodies 58 and 59. The electrodes on the top surface of the piezoelectric bodies 58 and 59 are bonded to a weight 60, and are electrically connected to each other via the weight 60. The weight 60 performs the functions of electrically connecting the electrodes on the top surface of the piezoelectric bodies 58 and 59 and increasing the detection sensitivity by applying a weight from above to the piezoelectric bodies 58 and 59.
The electrodes (not shown) on the bottom surface of the piezoelectric bodies 58 and 59 are in direct contact with the signal pickup electrodes 56 and 57, respectively, and are electrically connected thereto.
The electrodes on the bottom surfaces of the piezoelectric bodies 58 and 59 and the signal pickup electrodes 56 and 57 are connected in the following way. The electrode on the bottom surface of the piezoelectric body 58 and the signal pickup electrode 56 are brought into direct contact with each other and bonded together by using a small amount of an insulating adhesive material so that the electrode on the bottom surface of the piezoelectric body 58 and the signal pickup electrode 56 are brought into direct contact with each other in a manner in which the adhesive layer does not have a substantial thickness. This structure can be obtained because the bottom surface of the piezoelectric body 58 and the electrode formed on the bottom surface of the piezoelectric body 58 are so rough that the protruding portion of the electrode on the bottom surface of the piezoelectric body 58 and the protruding portion of the signal pickup electrode 56 are brought into direct contact with each other while the remaining portion of the electrode on the bottom surface of the piezoelectric body 58 and the signal pickup electrode 56 are bonded together via the insulating adhesive material.
The reason why conduction is secured by performing bonding by using an insulating bonding agent and by bringing the electrode on the bottom surface of the piezoelectric body 58 and the signal pickup electrode 56 into direct contact with each other as described above is that the amount of insulating bonding agent used is reduced, thereby achieving a lower height of the acceleration sensor 51.
In the acceleration sensor 51, the acceleration which acts in the direction of the arrow A (see FIG. 15), that is, the acceleration which acts in a direction parallel to the main surface of the hybrid IC substrate 52, is detected. In this case, since the piezoelectric bodies 58 and 59 are fixed onto the hybrid IC substrate 52, as described above, even if a large acceleration is generated in a direction at right angles to the hybrid IC substrate 52, breakage of the acceleration detection element 53, peeling off thereof from the hybrid IC substrate 52, and others are not likely to occur.
In an acceleration sensor for an airbag and the like, there has been a demand for detection of the acting acceleration with greater accuracy. The conventional acceleration sensor 51 is capable of accurately detecting an acceleration acting in the shearing direction. However, there has been a further demand for improvements in reliability of conduction in an acceleration sensor for an airbag and the like.
Also, in the above-described acceleration sensor, it has been found by the inventors of the present invention that an excessive amount of stress is transmitted from the hybrid IC substrate to the acceleration detection element because of temperature changes, causing sporadic noise to be generated and that there is a risk that this may lead to a malfunction.